Research on the pathology caused to models of lung lipids by ozone, nitrogen dioxide and cigarette smoke is being investigated. All of these agents either are themselves free radicals or cause the formation of free radicals in polyunsaturated fatty acids (PUFA). Ozone both shortens the induction period for autoxidation of lipids and also causes ozonolysis. Nitrogen dioxide initiates autoxidation of PUFA and also abstracts hydrogen from PUFA to form nitrous acid. Tobacco smoke contains long-lived free radicals both in the gas phase and in the particulate matter that is trapped on filters.